Transceivers

ABSTRACT

According to the present invention a common channel duplex a.m. transceiver comprises a pair of multiplicative mixers arranged to receive signals from two sources, receiver aerial means constituting one of the sources and arranged to feed received signals to the multiplicative mixers, transmitter aerial means, and oscillator/modulator means constituting the other of the sources and arranged to feed signals for transmission to the transmitter aerial means and to provide for the mixers of the pair, local oscillator signals which are at the same frequency as the carrier frequency for transmission and wherein it is arranged that the carrier frequency for transmission and the carrier frequency of the received signal are common to a single channel of the transceiver, phase quadrature means connected to provide a phase quadrature relationship between two signals fed to the mixers of the pair from one of the said sources, a pair of signal combiners fed one from each mixer, a pair of low-pass filters fed one from each combiner, a pair of correlator arrangements each having first and second input ports and an output port, the first input port of each correlator arrangement being fed one from each low-pass filter, detector means via which signals from the oscillator/modulator means are fed to respective second input ports of the correlators, the correlator arrangements being operative to correlate signals derived from the low-pass filters with detected signals from the oscillator/modulator means thereby to provide at the said output ports feedback signals which are fed one from each correlator to respective combiners, and an a.m. demodulator responsive to signals fed from the low-pass filters for providing a.m. output signals corresponding a.m. modulation carried by the received signals.

This invention relates to common channel duplex transceivers.

A common channel duplex transceiver is a transmitter/receiver which iscapable of receiving signals and transmitting signals contemporaneouslyon the same channel. A common channel duplex (CCD) transceiver suitablefor f.m. operation is described in our U.K. Patent Application No.10360/76. The present invention is however concerned more especiallywith an a.m.C.C.D. transceiver.

According to the present invention a common channel duplex a.mtransceiver comprises a pair of multiplicative mixers arranged toreceive signals from two sources, receiver aerial means constituting oneof the sources and arranged to feed received signals to themultiplicative mixers, transmitter aerial means, andoscillator/modulator means constituting the other of the sources andarranged to feed signals for transmission to the transmitter aerialmeans and to provide for the mixers of the pair, local oscillatorsignals which are at the same frequency as the carrier frequency fortransmission and wherein it is arranged that the carrier frequency fortransmission and the carrier frequency of the received signal are commonto a single channel of the transceiver, phase quadrature means connectedto provide a phase quadrature relationship between two signals fed tothe mixers of the pair from one of the said sources, a pair of signalcombiners fed one from each mixer, a pair of low-pass filters fed onefrom each combiner, a pair of correlator arrangements each having firstand second input ports and an output port, the first input port of eachcorrelator arrangement being fed one from each low-pass filter, detectormeans via which signals from the oscillator/modulator means are fed torespective second input ports of the correlators, the correlatorarrangements being operative to correlate signals derived from thelow-pass filters with detected signals from the oscillator/modulatormeans thereby to provide at the said output ports feedback signals whichare fed one from each correlator to respective combiners, and an a.m.demodulator responsive to signals fed from the low-pass filters forproviding a.m. output signals corresponding to a.m. modulation carriedby the received signals.

The common channel duplex a.m. transceiver may include a delay devicevia which signals from the oscillator/modulator means are fed to themixers of the pair, the delay introduced by the said device being chosento compensate for delay experienced by signals fed back from thetransmitter aerial to the receiver aerial whereby cancellation of theseunwanted signals in the combiners is optimised.

The correlator arrangements may each comprise first and secondmultiplicative mixers each of the said first and second multiplicativemixers having two input terminals and an output terminal, one inputterminal of each of the said mixers being coupled to the said firstinput port, the other input terminal of each mixer of the correlatorarrangement being coupled to the second input port, differentiator meansvia which the said other input terminal of the first mixer is coupled tothe second input port, first and second low-pass filters fedrespectively from the output terminals of the first and second mixers ofthe correlator arrangement and first and second voltage controllablepotential divider means each having a control signal input terminal andtwo other terminals, the control signal input terminal of the said firstand second voltage controllable potential divider means being fed fromthe first and second filters respectively, the first said potentialdivider means being connected by means of its said other terminalsbetween the said other input terminal of the said first mixer of thecorrelator arrangement and the output port thereof and the said secondpotential divider means being similarly connected between the saidsecond mixer of the correlator arrangement and the output port of thecorrelator arrangement.

The voltage controllable potential divider means may comprise a voltagevariable resistor connected effectively in series with a 180° phaseshift device, the voltage variable resistor and the said phase shiftdevice being connected in parallel with a further resistor, the controlsignal input terminal forming part of the voltage variable resistor andthe parallel combination of the resistor and the voltage variableresistor and the 180° phase shift means including the two otherterminals which are provided one at each end of the said parallelcombination.

The a.m. demodulator may comprise two squarers fed respectively from thesaid pair of low-pass filters, output signals from the squarers beingfed to a signal combiner an output signal from the signal combiner beingfed to a square rooting device which provides an a.m. output signalcorresponding to a.m. modulation carried by the received signals.

Alternatively, the a.m. demodulator may comprise two full-waverectifiers fed respectively from the said pair of low-pass filters andarranged to feed a signal combiner which provides a.m. output signalscorresponding to a.m. modulation carried by the received signals.

The phase quadrature means may be connected so that the local oscillatorsignals fed from the oscillator/modulator means to the pair ofmultiplicative mixers are in phase quadrature.

Alternatively the phase quadrature means may be connected so thatreceived signals fed from the receiver aerial means to the pair ofmultiplicative mixers are in phase quadrature. It should be understoodthat the phase quadrature means includes any device or arrangement forproducing a phase quadrature relationship between the signals fed to themixers and for example it is envisaged that the phase quadraturerelationship may be achieved by suitable spacing of two aerials so thatreceived signals fed to the pair of multiplicative mixers bear a phasequadrature relationship to each other.

Although the present invention is concerned especially with an a.m.common channel duplex transceiver, it is envisaged that a system forreceiving a.m. and f.m. may be provided wherein the f.m. system is asystem as of the kind described in our U.K. patent application No.10360/76.

Some embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings in which:

FIG. 1 is a generally schematic block diagram of a common channel duplexa.m. transceiver;

FIG. 2 is a generally schematic block diagram of an a.m. demodulator foruse with the transceiver of FIG. 1; and wherein

FIG. 3 is a generally schematic block diagram of an alternative a.m.demodulator for use with the transceiver of FIG. 1.

Referring now to FIG. 1 and a.m. transceiver comprises a pair ofmultiplicative mixers 1 and 2 fed with received signals from an aerial3. Local oscillator signals for the mixers 1 and 2 are provided by alocal oscillator 4 arranged to feed the mixers in phase quadrature via amodulator 5, a delay device 6 and a phase quadrature device 7. Themodulator 5 serves as a power amplifier for signals from the oscillator4 and receives a modulating signal on line 8 from a suitable source ofa.m. modulation. Modulated output signals from the modulator 5 areradiated from the transceiver via a transmitter aerial 9. Output signalsfrom the mixers 1 and 2 are fed to combiners 10 and 11 respectivelywhich are arranged to feed low-pass filters 12 and 13 respectively.Output signals from the low-pass filters 12 and 13 are fed to amplifiers14 and 15 respectively which are connected to feed an a.m. directconversion detector or demodulator 16. An a.f. output signalcorresponding to a.m. modulation received at the aerial 3 is providedfrom the demodulator 16 on line 17.

In order to cancel unwanted feedback, which is unavoidably fed back tothe receiver aerial 3, from the transmitter aerial 9, a modulationsample from the modulator 5 is correlated with the received signalpresent at the output of the amplifiers 14 and 15 and suitable feedbacksignals are developed which are applied on lines 18a and 18b tocombiners 11 and 10 respectively. Since the combiners 10 and 11 are fedfrom similar correlator arrangements only one of such arrangements isshown in FIG. 1. The correlator arrangement is shown within broken line19 and comprises two correlators 20 and 20'. The correlator 20 isidentical with the correlator 20' apart from the fact it is fed at oneinput via a differentiator 21 and therefore corresponding parts of thecorrelators 20 and 20' have been given the same numerical designationsdistinguished only by a dash suffix.

The correlator 20' comprises a mixer 22' fed at one input terminal 23'with a signal from the output of the amplifier 15 and at the other inputterminal 24' with a signal from the modulated 5 which is fed via adetector 25 and a smoothing capacitor 26. Thus the signal fed viaterminal 24' to the mixer 22' corresponds to the a.m. modulation on line8, and is correlated with the output signal from the amplifier 15.Output signals from the mixer 22' are fed via a filter 27' to controlthe resistance of a voltage variable resistor 28'. The voltage variableresistor 28' is connected in series with a 180° phase shift device 29',which might conveniently comprise a transformer, and the serialcombination of the parts 28' and 29' are connected in parallel with afurther resistor 30' to form a voltage sensitive potential dividerarrangement which is connected to feed the combiner 11 via line 18a andto receive signals from the detector 25 via the capacitor 26. Thecorrelator 20 of the correlator arrangement shown within the broken line19, serves also to provide a feedback correction signal on the line 18abut by reason of the fact that multiplicative mixer 22 is fed fromdetector 25 via the differentiator 21 a phase quadrature correctionsignal is provided. It is envisaged that the correlator arrangement 19may for some purposes comprise only those parts shown which bear thedashed suffix but for the most efficient unwanted signal cancellationthe full correlator arrangement shown within broken line 19 is required.

As hereinbefore explained the combiner 10 is fed via line 18b in asimilar manner to the combiner 11 thereby to provide suitablecancellation of unwanted feedback in the output lines from the mixers 1and 2.

The direct conversation a.m. detector or demodulator 16 may comprise apair of squarers 31, 32 as shown in FIG. 2 which are fed from theamplifiers 14 and 15 respectively. Output signals from the squarers 31and 32 are fed to an additive combiner 33 which in turn is arranged tofeed a square rooting device 34, an output signal from the squarerooting device being provided on a line 35 which provides an audiofrequency output signal corresponding to the a.m. modulation received atthe aerial 3. It will be appreciated that by reason of the operation ofthe correlators 20 and 20' in combination with the combiners 10 and 11feedback between the aerials 9 and 13 is cancelled.

Alternatively the demodulator 16 may comprise a pair of full-waverectifiers 36 and 37 which are arranged to feed an additive combiner 38,an output signal being provided on output line 39 from the additivecombiner 38. The full-wave rectifiers 36 and 37 are fed from theamplifiers 14 and 15 respectively as shown in FIG. 1 and the full-waverectifiers 36, 37 may be identical. The full-wave rectifiers may takeany convenient form and as shown in the drawing they may comprise forexample a pair of diodes 40, 41 and a phase shift device 42 providing a180° phase shift. Conveniently the phase shift device may comprise atransformer.

The transceiver hereinbefore described enables an a.m. signal to bereceived and transmitted on the same frequency channel. Signals whichare fed back from the transmission aerial 9 to the receiving aerial 3are suppressed by the injection of A.F. cancelling waveforms which arefed to the combiners 10 and 11. Because cancellation takes place at lowfrequency or audio frequency a very high degree of suppression of theunwanted feedback signal is achieved.

The audio signal outputs at the amplifier 14 and 15 are applied to apair of correlators such as the correlator 19 which receives referencesignals derived from the transmitted modulation. These unwanted signalspresent at the output of the amplifiers 14 and 15 which are due tofeedback between the aerials 9 and 13, is highly correlated with areference signal derived from the modulator 5 due to the fact that avery small delay occurs in the transmitter received demodulator path. Onthe other hand the wanted received signal is virtually uncorrelated withthe re-transmitted signal. The outputs from the correlators 20 and 20'after low-pass filtering apply slowly varying d.c. levels, which are ineffect a measure of the level of the unwanted signal at the output fromthe amplifiers 14 and 15, and these levels are used to control the leveland phase of the cancelling waveforms fed to the combiners 10 and 11.

It will be appreciated that in order to provide a radio relay functionreceived modulation on line 17 may be fed back via broken line 8a tomodulator 5 for re-transmission from the aerial 9.

For radio relay operation a delay device 43 is connected between theline 17 and the line 8 so that correlation of the wanted signal isdestroyed whereby the unwanted signal only is cancelled in the combiners10 and 11.

Various modifications may be made to the arrangement shown withoutdeparting from the scope of the invention and for example the phasequadrature device 7 may be connected alternatively to produce a phasequadrature relationship between the received signals applied to themixers 1 and 2 from the aerial 3.

What we claim is:
 1. A common channel duplex a.m. transceiver comprisinga pair of multiplicative mixers arranged to receive signals from twosources, receiver aerial means constituting one of the sources andarranged to feed received signals to the multiplicative mixers,transmitter aerial means, and oscillator/modulator means constitutingthe other of the sources and arranged to feed signals for transmissionto the serial means and to provide for the mixers of the pair localoscillator signals which are at the same frequency as the carrierfrequency for transmission, and wherein it is arranged that the carrierfrequency for transmission and the carrier frequency of the receivedsignals are common to a single channel of the transceiver, phasequadrature means connected to provide a phase quadrature relationshipbetween two signals fed to the mixers of the pair from one of thesources, a pair of signal combiners fed one from each mixer, a pair oflow pass filters fed one from each combiner, a pair of correlatorarrangements each having first and second input ports and an outputport, the first input port of each correlator arrangement being fed onefrom each low pass filter, detector means via which signals from theoscillator/modulator means are fed to respective second input ports ofthe correlators, the correlator arrangements being operative tocorrelate signals derived from the low pass filters with detectedsignals from the oscillator/modulator means thereby to provide at thesaid output ports feedback signals which are fed one from eachcorrelator to respective combiners, and an a.m. demodulator responsiveto signals fed from the low pass filters for providing a.m. outputsignals corresponding to a.m. modulation carried by the receivedsignals.
 2. A common channel duplex a.m. transceiver as claimed in claim1 including a delay device via which signals from theoscillator/modulator means are fed to the mixers of the pair, the delayintroduced by the said device being chosen to compensate for delayexperienced by signals fed back from the transmitter aerial to thereceiver aerial whereby cancellation of unwanted signals in thecombiners is optimised.
 3. A common channel duplex a.m. transceiver asclaimed in claim 1 or claim 2 wherein the correlator arrangements eachcomprise first and second multiplicative mixers, each of the said firstand second multiplicative mixers having two input terminals and anoutput terminal, one input terminal of each of the said mixers beingcoupled to the said first input port, the other input terminal of eachmixer of the correlator arrangement being coupled to the second inputport, differentiator means via which the said other input terminal ofthe first mixer is coupled to the second input port, first and secondlow pass filters fed respectively from the output terminals of the firstand second mixers of the correlator arrangement and first and secondvoltage controllable potential divider means each having a controlsignal input terminal and two other terminals, the control signal inputterminal of the said first and second voltage controllable potentialdivider means being fed from the first and second filters respectively,the first said potential divider means being connected by means of itssaid other terminals between the said other input terminal of the saidfirst mixer of the correlator arrangement and the output port thereofand the said second potential divider means being similarly connectedbetween the said second mixer of the correlator arrangement and theoutput port of the correlator arrangement.
 4. A common channel duplexa.m. transceiver as claimed in claim 3 wherein the voltage controllablepotential divider means comprises a voltage variable resistor connectedeffectively in series with a 180° phase shift device, the voltagevariable resistor and the said phase shift device being connected inparallel with a further resistor, the control signal input terminalforming part of the voltage variable resistor and the parallelcombination of the further resistor and the voltage variable resistorand the 180° phase shift means including the two other terminals whichare provided one at each end of the said parallel combination.
 5. Acommon channel duplex a.m. transceiver as claimed in claim 2 wherein thea.m. demodulator comprises two squarers fed respectively from the saidpair of low pass filters, output signals from the squarers being fed toa signal combiner, an output signal from the signal combiner being fedto a square rooting device which provides an a.m. output signalcorresponding to a.m. modulation carried by the received signals.
 6. Acommon channel duplex a.m. transceiver as claimed in claim 2 wherein thea.m. demodulator comprises two full-wave rectifiers fed respectivelyfrom the said pair of low pass filters and arranged to feed a signalcombiner which provides a.m. output signals corresponding to a.m.modulation carried by the received signals.
 7. A common channel duplexa.m. transceiver as claimed in claim 2 wherein the phase quadraturemeans is connected so that the local oscillator signals fed from theoscillator/modulator means to the pair of multiplicative mixers are inphase quadrature.
 8. A common channel duplex a.m. transceiver as claimedin claim 2 wherein the phase quadrature means is connected so thatreceived signals fed from the receiver aerial means to the pair ofmultiplicative mixers are in phase quadrature.